Feedback may occur in hearing aids when a loop exists between an audio input transducer, e.g. a microphone, and an audio output transducer, e.g. a loudspeaker or receiver. An audio signal received by the microphone is amplified and transmitted to the loudspeaker, but the sound from the loudspeaker can then be received by the microphone again, amplified further and then transmitted out through the loudspeaker again. This can result in a howl which may be very unpleasant for the hearing aid user and for other people in the surroundings. Furthermore, feedback can decrease the hearing aid user's sound perception. There are different ways to reduce feedback in hearing aids, e.g. by means of changing the phase of the frequency bands of an audio signal.
U.S. Pat. No. 6,876,751 presents a method for band-limited feedback cancellation. The cancellation is limited to a frequency band encompassing all unstable frequencies.
WO04105430 relates to oscillation suppression. A randomly changing phase is applied to the signal in one or more of several frequency bands based on whether oscillation is detected or suspected in the signal or not.
US2005/0226447 relates to oscillation reduction by phase shifting.
US2005/0047620 describes a hearing aid circuit comprising a phase shifter for feedback reduction.
US 2006/291681 A1 deals with a hearing aid comprising an adaptive feedback suppression system. The hearing aid comprises a pair of equalization filters having a frequency selection unit for respectively selecting from the processor input and output signals a plurality of frequency band signals and a frequency equalization unit for frequency equalizing the selected frequency band signals, and an adaptive feedback estimation filter for adaptively deriving the feedback cancellation signal from the equalized frequency band signals.
It remains a problem to improve feedback reduction in hearing aids in order to further improve the sound perception for hearing aid users.